A nephelometric turbidimeter determines the concentration of solid particles suspended in a liquid sample within a sample cuvette by projecting a light beam into the liquid sample within the cuvette. A light detector detects the amount of light scattered by the suspended solid particles at an angle of at least 30° to the light beam axis, for example, at a right angle to the light beam axis. Not only particles suspended in the liquid sample generate scattered light, however, any optical contamination of the cuvette, such as a fouling layer on the inside of the transparent cuvette wall or scratches at the cuvette body, may also scatter light.
US 2011/0043807 A1 describes a turbidimeter with two separate light sources and two separate light detectors so that different turbidity measurements under different scattering angles and with different penetration distances can be provided. The results of the different turbidity measurements are checked for plausibility, and a warning signal is generated if the different turbidity measurements do not seem plausible. The technical effort is relatively high and strong inhomogeneities of turbidity within the liquid sample can also cause non-plausible measurement results.